The present invention relates to a directional optical waveguide coupler, and more particularly, to a directional optical waveguide coupler wherein at least a portion of the light energy propagating in a direction of interest in the core of a cladded optical waveguide is coupled into a selected region of the cladding, which region is isolated from spurious cladding modes, and then coupled out of the selected cladding region for subsequent conversion to a representative electrical signal.
In future optical fiber transmission systems, it is expected that arrangements will be required for extracting signal wave information from optical waveguides. To monitor and control the transmission through an optical waveguide link, for example, it may be required to sample the signal propagating through the individual optical waveguides periodically along the link. Optical data bus links will require that signals be extracted for utilization at numerous selected points along the link. In most instances, it would be desirable if a portion of the signal propagating through the fiber could be tapped therefrom without breaking or terminating the fiber. Fiber terminations can add unwanted optical losses to the system, and would unfavorably increase the need for highly precise splicing and interconnecting arrangements.
Arrangements for tapping signal power from an intermediate portion of an optical fiber waveguide without requiring that the fiber be terminated or broken are known. For example, in patent application Ser. No. 522,518 of W. M. Muska, now U.S. Pat. No. 3,936,631, there is disclosed an arrangement wherein power is coupled out of the fiber waveguide by a dielectric body disposed in a coupling relationship with an intermediate length of the fiber, and is converted to a representative electrical signal suitable for utilization by a photodetector disposed adjacent to the dielectric body. To tap cladded fiber waveguides, all or most of the outer cladding is removed from the fiber in the vicinity of the fiber tap so that the dielectric body of the tap can extract power directly from the inner core. The dielectric body of the tap is disposed at least within about three optical wavelengths of the inner core to achieve the desired coupling relationship. Alternatively, the fiber is bent in the vicinity of the fiber tap to cause a portion of the optical power to radiate out of the inner core into the outer cladding from which it can be extracted by the dielectric body. In either case, some fraction of the power is tapped from the fiber, provided the index of refraction of the dielectric body is approximately equal to, or greater than, the index of refraction of the outer cladding of the fiber.
In patent application Ser. No. 522,519 of S. E. Miller, now U.S. Pat. No. 3,931,518, the above-cited Muska arrangement is improved to avoid removing most or all of the cladding by combining therewith means for coupling optical power propagating in the core of an optical waveguide into cladding modes. The mode coupling means illustratively takes the form of one or more corrugated plates which are pressed against the fiber to periodically deform a region of the fiber just preceding the fiber tap. Provided the spatial periodicity of the corrugations in the plates is properly selected, and the pressure exerted on the plates is sufficiently large, the periodic deformation of the fiber causes the desired higher order mode coupling, and assures that there is sufficient power distributed in the higher order modes of the fiber for the dielectric coupling body of the fiber to couple out. By adjusting the pressure that the plates exert against the fiber, it is possible to adjust the degreee of higher order mode coupling that takes place in the fiber. The above-mentioned Miller arrangement, however, does not prevent spurious cladding modes from reaching the energy tap, and therefore, cannot be considered a directional energy tap.
Prior art directional energy taps have generally included two separate optical waveguides positioned within an intermediate medium and arranged relatively close and substantially parallel to each other. Light propagating in a first direction in one optical waveguide is partially or fully transferred to the other optical waveguide by either the existence of a weak coupling between the two waveguides through the intermediate medium, or the application of electric or magnetic fields to change the coupling between the guides in the region of the intermediate medium. In this regard, see, for instance, U.S. Pat. Nos. 3,408,131, issued on Oct. 29, 1968 to N. S. Kapany; 3,558,213 and 3,589,794, issued on Jan. 26, 1971 and June 29, 1971, respectively, to E. A. J. Marcatili; and 3,859,503, issued on Nov. 26, 1974 to L. A. Riseberg et al.